This invention relates to printing inks and more particularly to printing inks suitable for use in jet drop printers of the type disclosed in Van Breemen et al, U.S. Pat. No. 4,080,607, and jet drop copiers of the type disclosed in Paranjpe et al, U.S. Pat. No. 4,112,469. Such printers print on high speed moving webs or on print media supported on a rotary drum by selective charging and catching of drops generated at a frequency on the order of about 120 KHz by streams of drops issuing from 500 or more orifices. These orifices have a diameter of less than 2 mils, and in some instances, less than 1 mil, which requires the printing ink to be free of anything but the very smallest particulate matter. Generally, the ink must be operable after filtering through a 3 micron filter and preferably should be able to undergo filtering through a 0.54 micron filter.
Further, for use in such printers the ink must be electrically conductive and have a resistivity below about 1,000 ohm cm, and preferably below about 500 ohm cm. For good runnability through small orifices, the ink should have a viscosity in the range between about 1 to 10 centipoise at 25.degree. C. In addition, the ink must be stable over a long period of time, compatible with the materials comprising the orifice plate and ink manifold, free of living organisms, and functional after printing. The functional characteristics after printing include smear and fade resistance, fast drying on paper (i.e., less than one second), and waterproof when dry.
When the ink is to be used in a jet drop copier, one other important property is required. Because start-up in a jet drop copier must be essentially instantaneous, ink is maintained in the ink supply manifold of the printing head for extended periods of time (e.g., up to two weeks). Therefore, an important property of an ink to be used in a jet drop copier is that it exhibit a very low vapor pressure and will not dry out when in the printing head.
It will be appreciated that it is quite difficult to provide an ink which performs as required after printing and yet is suitable for use in jet drop printers and copiers. Generally speaking, waterbase inks have been found to be most suitable because of their conductivity and viscosity range. However, even these ink formulations have tended to be a compromise between required properites. For example, several prior art water-base inks were not suitable for use in jet drop printers because they dried and tended to crystalize in and clog the small orifices in such printers. Use of organic solvent-base inks presented worse drying problems than those of aqueous base inks, and were undesirable for use in jet drop printers because of high evaporation rates, flammability, odor, and toxicity problems. It was discovered that humectants such as glycerine could be added to waterbase inks to reduce their tendency to dry out, but this increased the viscosity of such inks to the extent that they were unsuitable for rapid flow through the small orifices.
Improvements have been made, however. Meyer et al, U.S. Pat. No. 3,889,269, teaches that the addition of a hydroxyalkyl formamide to a waterbase jet printing ink prevents premature drying of the ink without an increase in viscosity. Edds et al, U.S. Pat. No. 3,870,528, discloses a combination of dyes which are taught to be stabilized against agglomeration by the addition of polyethylene glycol and N-methyl-2-pyrrolidone or diethylene glycol monoethyl ether. Ostergren et al, U.S. Pat. No. 3,846,141, discloses the addition of a mixture of a lower alkoxytriglycol and polyethylene glycol, a lower alkyl ether of diethylene glycol, or glycerol as a humectant in a jet printing ink. Both Zabiak, U.S. Pat. No. 3,705,043, and British Pat. No. 1,494,768, teach the addition of alkylene glycols and/or alkyl ethers of alkylene glycols as humectants to jet printing ink compositions. Finally, German published patent application No. 2,555,296, teaches a waterbase jet printing ink composition containing a water insoluble dyestuff and an organic chemical solvent solubilized by a complex former consisting of a diethanol or triethanolamine salt of a long-chain fatty acid and an aromatic anionic wetting agent.
However, such inks have not been entirely satisfactory in use in jet drop printers and particularly in jet drop copiers which require an ink which will not dry out in the printing head for extended periods of time. Accordingly, the need still exists in the art for a jet drop printing ink composition which will not agglomerate or dry out in the print head and yet which drys fast once printed and is smudge and water resistant.